1. Field of the Invention
This invention relates to a transmission-line transformer having a seat member, and to a double-balanced mixer employing such a transformer primarily for use in a CATV tuner.
2. Description of the Related Art
Since the double-balanced mixer used in an up-down-converter for a CATV tuner must maintain characteristics thereof over a wide band, several transmission-line transformers are generally used therein.
One example of such a double-balanced mixer is shown in FIG. 2 wherein four diodes 11.about.14 are connected effectively in a bridge arrangement between a transmission-line transformer 22 and LC phase distributors 31, 32. Numerals 21, 23 designate input/output transmission-line transformers which are generally disposed as shown.
By means of transmission-line transformer 22, diodes 11 .about.14 and phase distributors 31, 32, the high-frequency signal RF transmitted to terminal 41 is subject to frequency conversion based on local oscillating signals LO transmitted to terminal 42, and output to terminal 43 is the intermediate-frequency signal IF.
FIG. 3(A) and FIG. 3(B) show conventional structures for the transmission-line transformers and for the double-balanced mixer employing such transformers.
Conventional transmission-line transformers have a construction in which a plurality of thin windings 52 are wound around annular cores 51 to form transmission-line transformers 50a.about.50c.
In the conventional double-balanced mixer, the transmission-line transformers 50a-50c are mounted on a circuit board 2 (for example, a printed substrate), and lead portions 53 of each winding 52 are passed through holes 3 formed in the circuit board 2 to be soldered to an electrode pattern (not shown) which is disposed on the reverse side of the circuit board 2, thereby fixing and electrically connecting each of transmission-line transformers 50a.about.50c.
In the double-balanced mixer described above which employs the conventional transmission-line transformers 50a .about.50c, a thin wire is used for the windings 52 whereby a problem occurs in that, when each of the transmission-line transformers 50a.about.50c is fixed to the circuit board 2, each lead portion 53 of the windings 52 must manually be passed through each hole 3 in the circuit board 2 using a pair of tweezers, thereby requiring much assembling time.
Another problem is also posed in that each of transmission-line transformers 50a.about.50c on the circuit board 2 fluctuates in position, and the lead portions 53 thereof also fluctuate in length and position, thus causing considerable variation in the double-balanced mixer characteristics after assembling thereof. Such variations in turn necessitate minute characteristic adjustments by altering intervals between the windings 52 and the like.